The drive for higher performance, higher density electronics has lead to continual scaling of the lateral dimensions of metal-oxide-semiconductor (MOS) devices. As lateral device dimensions are reduced, a MOS device's gate dielectric thickness (e.g., silicon dioxide thickness) also must be reduced to maintain sufficient charge storage capacity for proper operation of the MOS device.
Modern lateral device dimension requirements have forced gate dielectrics into the sub-40 angstrom regime without a proportional decrease in drive voltage. The combination of thinner gate dielectric layers with the same or similar drive voltages has lead to increased device electric fields for each successive MOS device generation. Accordingly, hot-carrier damage associated with these increased electric fields and dielectric breakdown strength have become major concerns with regard to further scaling of MOS devices. Additionally, reduced MOS device dimensionality has led to extensive use of fabrication techniques such as e-beam lithography and reactive ion etching which employ energetic particles and produce ionizing radiation that can damage conventional furnace grown silicon dioxide (SiO2) gate dielectrics.
An alternative to the use of “pure” silicon dioxide as a gate dielectric is the use of “nitrided oxides” or “oxynitrides”. An oxynitride typically incorporates a small amount (e.g., 1-5 atomic percent) of nitrogen at the Si/SiO2 interface. The interfacial nitrogen improves the hot-carrier and radiation damage resistance of oxynitrides, and enhances the oxynitride's barrier diffusion properties. Nitrogen in the bulk of the oxide layer increases the dielectric constant of the oxide layer.
One technique that may be used to introduce nitrogen to silicon dioxide is plasma nitridation. In plasma nitridation, an RF plasma reactor is employed to incorporate nitrogen into an oxide film. For example, an RF plasma may be employed to dissociate molecular nitrogen into atomic (and/or ionic) nitrogen, and the atomic and/or ionic nitrogen then may be incorporated into the oxide film. The plasma generally is employed without applying bias power. That is, the nitrogen plasma is primarily created through pulsed or continuous source (coil) power.
Following conventional plasma nitridation, similarly processed substrates may exhibit varying oxide thicknesses and/or nitrogen incorporation levels. Such varying oxide thicknesses and/or nitrogen incorporation levels may lead to undesirable device-to-device variations that affect device yield and cost. As such, improved methods and apparatus for incorporating nitrogen in oxide films are desirable.